The present invention is directed to compositions and methods for producing a therapeutic benefit by producing vascular occlusion using platelet activation as the initiating event. Compositions and methods of the invention involve targeting platelets to a site, such as a tumor site, using a binding agent, such as a bifunctional molecule, a portion of which binds to the site and another portion of which binds or immobilizes a platelet binding agent, such as circulating von Willebrand Factor (VWF).